Relaxation type oscillators have been used extensively for generation of ignition sparks in various types of fuel burning equipment. Many times these devices are referred to as silicon controlled rectifier spark generators. These devices utilize a capacitor that is charged from a potential source and then rapidly discharged by the gating of a silicon controlled rectifier so that the discharged current flows through the primary winding of a step up or high voltage transformer. This type of spark generator has been used extensively in automobile ignitions, gaseous fuel burner ignition systems, and in oil burner ignition systems.
Typically a relaxation type of oscillator relies on a circuit that allows for the energy storage capacitor of the device to be charged on one half cycle of the applied alternating current, and then provides a gating signal or pulse on the reverse half cycle to discharge the capacitor. This type of spark generation has been very reliable and is inexpensive. The concept of charging the energy storage capacitor on one half cycle of the applied alternating current and then discharging it on the reverse half cycle has certain drawbacks and disadvantages that must be overcome. In this type of a device, line voltage transients are present on both half cycles of the applied voltage and they can seriously interfere with other equipment that is operated in conjunction with the spark generator. More specifically, if a relaxation oscillator type of spark generator is utilized with a rectification type of flame sensor, the transients of current in the system on both half cycles can interfere with or simulate the presence of flame when that is not desirable. Also, there are certain types of ultraviolet sensing systems that are operated with the ultraviolet sensor active on one half cycle and the spark generator active on the reverse half cycle. In this type of a system it is undesirable to have line transients present on both half cycles of operation of the device.
These deficiencies have been recognized by others and there are a few circuits which disclose charging of a capacitor for use in a relaxation oscillator type of spark generator on the same half cycle as the firing of the associated silicon controlled rectifier. These circuits are rather complex, costly, and in certain cases the circuitry does not provide a good driving signal for the silicon controlled rectifier. Certain of the prior art devices that have been used to operate an ignition device on the same half cycle of the operation of the device as the generation of spark tend to have a deficiency in the manner in which the silicon controlled rectifier is gated and this deficiency is known as "gate starvation". Gate starvation is a situation in which the rise of the gate potential is relatively slow and does not cleanly drive the associated silicon controlled rectifier into a full "on" condition in a short period of time.